Spacers are utilized in nuclear fuel bundles to maintain the required spacing between vertically upstanding, laterally spaced, side-by-side nuclear fuel rods within the bundles. As well known, a nuclear fuel bundle contains an array of a predetermined number of fuel rods, for example, 8.times.8, 9.times.9 or 10.times.10 arrays of fuel rods. The fuel rods are supported on a lower tie plate and extend upwardly to an upper tie plate and, in a BWR, within a fuel bundle channel. The primary function of the spacers is to align the fuel rods in correct side-by-side relation to one another throughout their vertical length.
Spacers are commonly constructed having two constituent parts. The first part includes an interior grid formed of an array of discrete unit cells. Each unit cell surrounds a fuel rod and the cells are typically welded to one another to form a grid. The second constituent part comprises a band about the spacer which locates the spacer within the fuel bundle channel in a BWR. A typical unit cell includes two spring legs spaced from one another and connected at the top by a fuel rod encircling arm and at the bottom by another fuel rod encircling arm. Each spring leg is deflected inwardly and defines at a central portion thereof a respective rod contacting portion. The pair of spring legs are orthogonally related and bias the rod against stops formed opposite the spring legs in the upper and lower encircling arms. The upper and lower encircling arms are generally of an octagonal configuration, although the stops are provided by arcuately inwardly directed portions of those arms opposite the spring legs.
To fabricate a complete spacer, the unit cells and bands are typically placed in a fixture, which holds them in the correct positions for welding. The welds are formed where the adjacent cells abut, i.e., at each of the four linearly orthogonally related sides of each of the upper and lower encircling bands. This fabrication procedure, however, is slow, labor-intensive and not adaptable to automation. The basic problem is that the individual unit cells are readily and easily damaged. For example, the upper and lower encircling arms are, in effect, cantilevered horizontally from the vertically extending springs. These encircling arms can thus be bent easily during assembly. A typical assembly procedure employs a jig comprised of a base plate with pins projecting upwardly from the plate. To align the unit cells before welding, the unit cells are disposed on the pins by sliding the unit cells axially onto the pins. The upper and lower arms of the unit cell are thus subject to bending during this insertion process. They also can hang up on adjacent cells.